Methoxymagnesium Chloride—Structure and Interaction with Electron

Article pubs.acs.org/JPCC

Methoxymagnesium ChlorideStructure and Interaction with Electron Donors: Experimental and Computational Study Ville H. Nissinen, Mikko Linnolahti, and Tuula T. Pakkanen* Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland S Supporting Information *

ABSTRACT: In this study, the structure of methoxymagnesium chloride (MgCl(OMe)) and its interaction with ester electron donors (diisobutyl phthalate and ethyl benzoate) and TiCl4 were studied using experimental (PXRD, DRIFT, CP/ MAS 13C NMR) and computational (DFT; M06-2X functional) methods. Alkoxymagnesium chlorides (MgCl(OR)) have been claimed in several patents to be potential support materials for Ziegler−Natta catalysts. We have found that MgCl(OMe) possesses a layered structure similar to that of MgCl2. An energy comparison among MgCl2, Mg(OMe)2, and MgCl(OMe) structures indicates that mixing of Cl and OMe, and thus the formation of methoxymagnesium chloride, is thermodynamically feasible. Computational results suggest an energetic preference for MgCl(OMe) being structurally composed of MgCl2 and Mg(OMe)2 units, which are arranged in a form of alternating stripes. Based on both spectroscopic and computational data, the coordination of ester electron donors on MgCl(OMe) is much weaker than on MgCl2. On the other hand, the coordination of TiCl4 is stronger in the case of MgCl(OMe). TiCl4 prefers to coordinate on the (110)-like surface of MgCl(OMe), whereas ester electron donors exhibit no particular preference for either the (110)-like or (104)-like surfaces. MgCl(OMe)/TiCl4 product was found to be an active catalyst in ethylene polymerization.

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INTRODUCTION Ziegler−Natta catalysis, one of the most important heterogeneous catalytic processes,1 is still a key player in the mass production of polyolefins.1,2 Modern Ziegler−Natta catalysts are composed of active species TiCl4, MgCl2 support, electron donors (Lewis bases), and alkyl aluminum cocatalyst.1,3 MgCl2 has been found to be a suitable support material for TiCl4 due to the similar crystal structures and lattice parameters of δMgCl2 and δ-TiCl3.4 The crystal structure of MgCl2 consists of Cl−Mg−Cl triple layers, which can be packed in a way that Cl atoms form distorted cubic close packed (α-MgCl2) or hexagonal close packed structures (β-MgCl2).5−7 From the catalyst point of view, activated δ-MgCl2 (characterized by a high structural disorder of Cl−Mg−Cl layers and a small crystallite size) is the most interesting form due to the high amount of unsaturated lateral surfaces.1,8,9 Electron donors can bind to the unsaturated Mg atoms on these surfaces and thereby have an influence on the catalytic properties of titanium centers.10−13 Interactions of MgCl2 with electron donors and TiCl4 have been extensively studied in the past decades using experimental and computational approaches.5,6,13−24 Although MgCl2 is by far the most employed support in Ziegler−Natta catalysts, there are other potential support materials, e.g., MgBr2,1 MnCl2,1 polymers,25,26 SiO2,15,27 and magnesium alkoxides (Mg(OR)2).28 Alkoxymagnesium chlorides (MgCl(OR)),29 in which one Cl of MgCl2 has been replaced with an OR group, are also interesting, yet sparsely © XXXX American Chemical Society

studied support materials. Synthetic pathways to alkoxymagnesium chlorides30−32 and their use as precursors or components of Ziegler−Natta catalysts29,33−36 have been reported mostly in the patent literature. Turova et al. have examined the synthetic routes leading to various alkoxymagnesium chlorides and characterized the products using IR spectroscopy and X-ray diffraction.37 Ethoxymagnesium chlorides, MgCl(OEt) and Mg2Cl3(OEt), prepared by Smith et al. were according to Xray diffraction and CP/MAS13C NMR spectroscopy homogeneous materials.29 Incorporation of TiCl4 to these ethoxymagnesium chlorides resulted in active ethylene polymerization catalysts. Although there is evidence for the use of alkoxymagnesium chlorides as supports for Ziegler−Natta type catalysts, no studies on structures of alkoxymagnesium chlorides and their interaction with electron donors have been reported. In our previous study,38 we have observed an unexpected formation of methoxymagnesium chloride, MgCl(OMe), in our synthesis of MgCl2-diether adducts according to the method earlier presented by Di Noto et al.39 MgCl(OMe) was formed through a two-step cleavage reaction of 1,3-dimethoxypropane diether induced by BuMgCl. In the present study, we examine the structural characteristics of methoxymagnesium chloride Received: June 27, 2016 Revised: September 9, 2016

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DOI: 10.1021/acs.jpcc.6b06488 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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A complexometric EDTA (ethylenediaminetetraacetic acid) titration was used to determine the magnesium content of the solid products. pH of the solution was adjusted with NH3/ NH4+ buffer, and Eriochrome Black T color indicator was used for the end point detection. The amounts of organic compounds in the complexes were determined by 1H NMR spectroscopy (Bruker Avance 400 spectrometer). The solid products were dissolved in 10% (V/V) D2SO4/D2O solution in order to decompose the complexes. Sodium acetate was used as an internal standard, the number of scans was 32, and relaxation delay was 10 s. The titanium content was determined by a spectrophotometric method40 in which solids were dissolved in H2SO4 solution, and due to addition of H2O2, a yellow complex was formed. Absorbance of the solution was read at 410 nm wavelength using a Shimadzu UVmini-1240 spectrophotometer. The determined mole fractions of magnesium, titanium, and organic moieties were used to calculate the chlorine content of the products. Ethylene Polymerization. To test performance of the catalysts in ethylene polymerization, approximately 20 mg of catalyst, 50 mL of n-heptane, and triethylaluminum were packed into a 100 mL autoclave in the glovebox. An Al/Ti molar ratio of 100:1 was used. The autoclave was heated to 50 °C and the ethylene feed (2 bar) was started after 30 min from the beginning of the heating. The polymerization was terminated after 60 min by stopping the ethylene feed and by adding a mixture of ethanol and hydrochloric acid. The polymer produced was washed with ethanol and dried at 60 °C overnight. Computational Details. All the calculations were performed using the Gaussian 09 program package.41 Both the cluster models and periodic models of magnesium chlorides and their donor adducts and TiCl4 complexes were studied by DFT methods using the M06-2X meta-hybrid GGA functional,42 which has been shown to be a cost-effective choice for systems that contain alkyls and/or halides bridging between main group metals.43 For the sake of periodic calculations, optimized triple-ζ-valence + polarization basis sets (TZVP) were used in all calculations.19 The basis sets have been derived from the def-TZVP basis sets of Ahlrics and co-workers.44 The cluster models were preoptimized by the hybrid density functional PBE0 method.45,46 Gibbs energies were calculated at T = 298 K and p = 1 atm. In the periodic calculations, the monolayers were described by multiplication of a Mg4Clx(OMe)y unit cell, where x + y = 8. The catalytically relevant surfaces were presented by one-dimensional ribbons having thickness of five atomic layers (see Figure SA1 in the Supporting Information). The model choice was based on our previous computational studies of MgCl2/TiCl4/donors.19−22 PBC module of Gaussian was adopted for periodic calculations with the default k-space integration method and PBC cells range of 100 Bohr.

and its potential as a support material for a Ziegler−Natta catalyst. Both experimental and computational methods are utilized in order to obtain a comprehensive overview of the structure of MgCl(OMe) and its interactions with TiCl4 and two typical electron donors, namely diisobutyl phthalate (DIBP) and ethyl benzoate (EB).

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EXPERIMENTAL SECTION Materials and General Considerations. Toluene (anhydrous, 99.8%), n-octane (reagent grade, 98%), 1-chlorobutane (ReagentPlus, 99%), ethylbenzoate (>99%), and titanium tetrachloride (ReagentPlus, 99%) were obtained from SigmaAldrich. Diisobutyl phthalate (for synthesis) and n-heptane (for analysis) were purchased from Merck and 1,3-dimethoxypropane (98%) from CHEMOS GmbH. 1,3-Dimethoxypropane was distilled before usage. The solvents, DIBP, EB, 1,3dimethoxypropane, and 1-chlorobutane were dried and stored over activated 3 Å molecular sieves. Magnesium turnings (Acros Organics, 99.9+ %) were dried at 110 °C at least for 2 days before use. A glovebox and Schlenk techniques were utilized in order to ensure oxygen and moisture free atmosphere. All the glass ware and other equipment used in the experiments were stored in an oven at 110 °C and rapidly transferred to the loading chamber of the glovebox when needed. Synthesis of MgCl(OMe). Methoxymagnesium chloride was synthesized using magnesium, 1-chlorobutane, and 1,3dimethoxypropane as starting materials. The synthetic procedure is described in detail in our previous publication.38 The final product, MgCl(OMe), is a fine white powder. Addition of DIBP, EB, and TiCl4 to MgCl(OMe). DIBP, EB, and TiCl4 were added to MgCl(OMe) in an autoclave using toluene as a solvent medium. For the addition of DIBP and EB, a donor/Mg molar ratio of 1:1 was used. Reagents were packed into the autoclave in the glovebox and then the autoclave was heated at 130 °C for 2 h. The product formed was separated by a filtration and washed with toluene in the glovebox. After washing, the product was dried in vacuum at room temperature for 2 days. For the addition of TiCl4 to MgCl(OMe), a Ti/Mg molar ratio of 10:1 was used. Reagents were packed into the autoclave, which was then heated at 100 °C for 2 h. The solid product formed was separated, washed with toluene and heptane and dried as in the cases of DIBP and EB. Characterization of the Products. Powder X-ray diffractograms were recorded with a Bruker AXS D8 ADVANCE diffractometer using Cu Kα radiation (λ = 1.5418 Å). The following parameters were used: a measurement range (2θ) of 4.0−70.0°, a step size of 0.05°, and a time per step of 8 s. Measurements were conducted with a custommade steel sample holder and Mylar film was used to protect the samples from air. CP/MAS (cross-polarization/magic angle spinning) 13C NMR spectra of the products were recorded with a Bruker AMX-400 spectrometer using the following parameters: a spin rate of 4500 Hz, a relaxation delay of 5 s, a contact time of 3.0 ms, and a number of scans of 10 000. Glycine was used as an external standard for calibrating the chemical shifts. A Nicolet Impact 400D spectrometer was utilized in recording the IR spectra of the solid products (number of scans of 32, resolution of 2 cm−1). Measurements were conducted using a DRIFT (diffuse reflectance infrared Fourier transform) unit mounted inside a glovebox to ensure inert atmosphere.

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RESULTS AND DISCUSSION Structure of MgCl(OMe). A reaction of magnesium, 1chlorobutane, and 1,3-dimethoxypropane (in a mole ratio of 2:3:1) results in formation of methoxymagnesium chloride.38 According to an X-ray diffraction study presented in Figure 1, methoxymagnesium chloride has a diffraction pattern similar to that of δ-MgCl2 prepared with the same synthesis method as MgCl(OMe) but in the absence of an electron donor. The Xray pattern of MgCl(OMe) exhibits three broad bands centered approximately at (2θ) 12°, 32°, and 54°, whereas the X-ray B

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resembles the tetrameric MgCl2 cluster.47 In order to examine the preferred distribution of Cl and OMe in the MgCl(OMe) clusters, the size of the cluster was further increased to construct a hexagonal structure48 composed of 19 monomers. The mutual arrangement of methoxy groups was varied and the lowest energy structure obtained for [MgCl(OMe)]19 shows that methoxy groups are located close to each other, resulting in formation of alternating stripes of Mg(OMe)2 and MgCl2 with an overall composition of MgCl(OMe) (Figure 2). The reaction energies and reaction Gibbs energies calculated as a function of the cluster size for the formation of MgCl(OMe) from MgCl2 and Mg(OMe)2 (Table 1) indicate that the formation of MgCl(OMe) by mixing of Cl and OMe is energetically and thermodynamically feasible.

Figure 1. Powder X-ray diffractograms of MgCl(OMe) (solid line) and δ-MgCl2 (dashed line). Reflections of the sample holder are marked with asterisks (*).

Table 1. Reaction Energy (ΔE/Mg) and Reaction Gibbs Energy (ΔG/Mg) for [MgCl2]x + [Mg(OMe)2]x ⇌ 2 [MgCl(OMe)]x

pattern of δ-MgCl2 shows three bands at (2θ) 15°, 32°, and 50°. The congruence between the X-ray patterns indicates that methoxymagnesium chloride and δ-MgCl2 possess a similar layer structure. The signals of MgCl(OMe) are wider than those of δ-MgCl2, indicating even a higher structural disorder. A wide signal of C−O stretching vibration in the DRIFT spectrum (Figure SB1) at 1000 cm−1 indicated multiple possible chemical environments of methoxy groups in MgCl(OMe). The wide signal of methoxy group in CP/MAS 13C NMR spectrum (Figure SB2) at 54 ppm supports the IR result. According to the NMR spectroscopic data, a minor amount of unreacted 1,3-dimethoxypropane is present in the product, probably bound to MgCl(OMe) surface. Alongside the experiments, the structure of MgCl(OMe) was studied by quantum chemical calculations in order to provide insight into the general structural features. First, clusters of up to four MgCl(OMe) monomers were studied in all possible configurations to make a detailed comparison to analogous small MgCl2 clusters. The obtained tetramer (Figure 2) closely

x

ΔE/Mg (kJ/mol)

ΔG/Mg (kJ/mol)

1 2 3 4 19

−3.8 −43.0 −31.0 −39.7 −22.1

−10.7 −41.5 −30.0 −38.0 −22.0

The results from cluster model calculations suggest that MgCl(OMe) forms a layered structure similar to that of MgCl2. To confirm this and to generalize the structural features for the analogous crystallites, systematic periodic quantum chemical calculations were carried out for MgCl(OMe) monolayer systems. The layers were described by multiplication of a Mg4Clx(OMe)y supercell, where x + y = 8. The lowest energy structure of each combination of x and y with energies relative to the pure MgCl2 layer (x = 8, y = 0) is given in Table SB1. The overall lowest energy structure is obtained at Cl:OMe ratio of 1:1 (x = 4, y = 4), which is shown in Figure 3 (top). In agreement with the cluster calculations reported above, the periodic calculations show the preferable mixing of Cl and OMe, which is maximized at the equivalent composition to give MgCl(OMe). The energy relative to the pure MgCl2 is −48.3 kJ/mol per each Mg. The periodic calculations indicate that in an optimized structure the MgCl2 and Mg(OMe)2 units are not randomly mixed but are arranged in a form of alternating stripes. However, most probably the synthesized methoxymagnesium chloride does not possess this neatly ordered structure throughout the whole material. In addition to a layer stacking disorder, there is probably also an intralayer disorder between Cl and OMe units, which would in part explain the observed disorder in PXRD. PXRD data (Figure 1) was used to evaluate the experimental interlayer distance of MgCl2 and MgCl(OMe) structures. The PXRD signal of δ-MgCl2 at 15° is associated with crystal plane (003) and describes the distance between adjacent triple layers in the crystallographic direction. Based on our X-ray analysis, the interlayer distances of δ-MgCl2 and MgCl(OMe) are 5.9 and 7.6 Å, respectively. Periodic calculations concerning adjacent triple layers (Figure 3 bottom, Table SB2) resulted in interlayer distances of 5.6 and 7.1 Å for MgCl2 and MgCl(OMe), respectively. Both the calculated values are approximately 5% shorter than the experimental values, which indicates that the employed M06-2X method somewhat overestimates the dispersion between adjacent layers. MgCl-

Figure 2. Most stable tetrameric [MgCl(OMe)]4 (top left) and [MgCl2]4 (top right) structures and hexagonal structure of [MgCl(OMe)]19 (bottom) from two viewpoints. Green = Cl, yellow = Mg, red = O, gray = C. C

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copy is known to be a powerful tool for investigating complexes of carbonyl compounds because the wavenumber of the CO stretching vibration is highly sensitive to coordination.6 The coordination of esters on unsaturated edges of magnesium chlorides may result in formation of different types of surface complexes. On this account, the CO stretching bands in the IR spectra of MgCl(OMe)/DIBP and MgCl(OMe)/EB complexes (Figures SC4 and SC5) were deconvoluted using Levenberg−Marquardt algorithm and Gaussian line shape (Figure 4).

Figure 3. Layer structure of MgCl(OMe). Top: single layer illustrated from orientation parallel to the basal (100) surface; bottom: multilayer structure.

(OMe) has a longer interlayer distance than MgCl2 due to a larger spatial requirement of methoxy group. Interaction of MgCl(OMe) with Electron Donors. Two typical ester electron donors (DIBP and EB) and TiCl4 were added to MgCl(OMe) in order to investigate the interaction of MgCl(OMe) with electron donors and TiCl4. According to chemical compositions of the reaction products (Table 2) the amounts of electron donors in the products are low compared to similar MgCl2/DIBP and MgCl2/EB complexes.6,14 On the other hand, Ti content of MgCl(OMe)/TiCl4 complex is of same order as in the case of MgCl2/TiCl4 complex. Based on PXRD analysis, the structure of MgCl(OMe) was not significantly affected by the addition of either electron donors or TiCl4 (Figure SC1). The IR and NMR studies (Figures SC2 and SC3) also show that addition of TiCl4 did not cause any major changes in the structure of MgCl(OMe), suggesting that TiCl4 does not react with the OMe groups of MgCl(OMe) under the reaction conditions used (see Table 2). However, some changes in composition in the local environment of TiCl4 adsorption sites might be possible. The infrared spectroscopic data indicated coordination of ester electron donors to MgCl(OMe) through carbonyl group. IR spectros-

Figure 4. Deconvoluted IR spectra of MgCl(OMe)/DIBP (top) and MgCl(OMe)/EB (bottom) complexes.

In the case of MgCl(OMe)/DIBP complex, the CO stretching band is composed of two superimposed absorptions at 1689 and 1725 cm−1 at almost equal proportions. For free DIBP, the CO signal is located at 1727 cm−1 giving Δν(C O) values of 2 and 38 cm−1 for MgCl(OMe)/DIBP complex. The absorption at 1725 cm−1 is associated with loosely bound or unbound DIBP, whereas the absorption at 1689 cm−1 is associated with DIBP complexed to unsaturated surfaces of MgCl(OMe). For MgCl2/DIBP/TiCl4 system three distinct absorptions with Δν(CO) values as high as 72 cm−1 have been reported.6 Potapov et al. have presented similar results for the MgCl2/dibutyl phthalate complex as three distinct

Table 2. Chemical Compositions (wt %) of MgCl(OMe) and the Products from Addition of TiCl4, DIBP, and EB to MgCl(OMe) product MgCl(OMe) MgCl(OMe)/TiCl4 MgCl(OMe)/DIBP MgCl(OMe)/EB

wt % (Mg) 24.5 23.9 23.1 24.6

wt % (Cl) 44.9 45.1 39.3 37.1

b

wt % (OMe)

wt % (dmp)a

25.6 24.6 26.9 26.5

5.0 4.3 3.5 3.6

wt % (Ti)

wt % (DIBP)

wt % (EB)

2.1 7.2 8.2

a Residual 1,3-dimethoxypropane present in the product. bCl content is based on calculations, and in the case of MgCl(OMe) it is overestimated due to residual solvent in the product.

D

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The Journal of Physical Chemistry C absorptions with Δν(CO) values of 29, 56, and 78 cm−1 were reported.14 In the case of the MgCl(OMe)/EB complex, the CO stretching band is also composed of two distinct absorptions at 1719 and 1692 cm−1. For free EB the signal of CO stretching vibration is located at 1719 cm−1 giving Δν(CO) values of 0 and 27 cm−1 for the MgCl(OMe)/EB complex. The absorption at 1719 cm−1 is associated with unbound EB, and the absorption at 1689 cm−1 is associated with EB complexed to MgCl(OMe). For the MgCl2/EB/TiCl4 system three distinct absorptions with Δν(CO) values of 28, 45, and 69 cm−1 have been reported.6 Also for the MgCl2/EB complex without TiCl4 similar results (Δν(CO) values of 23, 45, and 71 cm−1) have been presented.14 Significantly lower Δν(CO) values of MgCl(OMe)/electron donor complexes indicate notably weaker interactions of DIBP and EB with MgCl(OMe) compared to MgCl2. In addition to IR spectroscopy, CP/MAS 13C NMR spectroscopy was used to study the coordination of electron donors to MgCl(OMe). Due to the relatively low contents of electron donors in the MgCl(OMe)/electron donor complexes, the signals of carbonyl carbons in 13C NMR spectra (Figures SC6 and SC7) are weak. In the case of MgCl(OMe)/DIBP, the wide signal of carbonyl carbon is centered at 173.9 ppm and in the case of MgCl(OMe)/EB at 172.2 ppm. For unbound DIBP and EB positions of the carbonyl signals are 167.7 and 166.5 ppm, respectively. Thus, in both cases a downfield shift of approximately 6 ppm had occurred due to coordination. Coordination of TiCl4 and the electron donors on selected MgCl2 and MgCl(OMe) surfaces were studied by the periodic calculations. For the sake of making a practical comparison between MgCl2 and MgCl(OMe), we focused on the catalytically relevant (104) and (110) surfaces of MgCl2 and their analogous MgCl(OMe) counterparts, which are referred as (104)-like and (110)-like surfaces. Note that there are several ways to obtain (104)-like and (110)-like cuts in MgCl(OMe) due to the stripe structure and mixing of Cl and OMe. In the present work, the lateral cuts of the MgCl(OMe) surfaces were systematically cut so that MgCl2 and Mg(OMe)2 surface units alternate in the periodic direction (see Figure 5). Dimethyl phthalate was used as a model for DIBP to simplify the alkyl chain rotations. The optimized structures of the TiCl4/donor bound surfaces are illustrated in Figure 5. Energies are given in Table 3, including comparison to MgCl2. To account for the relative stabilities of the TiCl4/donor bound surfaces and to enable comparison between the (104)-like and (110)-like surfaces, the energies are reported relative to the fully saturated crystalline layer and per length of surface (Å).20 More details are available in Table SC1. Without TiCl4/donors, the formation of (104)-like and (110)-like lateral cuts of MgCl(OMe) is less favorable than the formation of corresponding lateral cuts of MgCl2. Similar to MgCl2,49 TiCl4 on MgCl(OMe) prefers octahedral sixcoordination, both on (104)-like and (110)-like surfaces, which in the case of (104)-like cut requires binuclear binding of TiCl4. The coordination of TiCl4 stabilizes MgCl(OMe) more than MgCl2, whereas the donors show an opposite effect, stabilizing MgCl(OMe) less than MgCl2, in accordance with the experimental data. TiCl4 shows preference for the (110)like surface of MgCl(OMe), whereas the donors do not show any particular preference for either (104)-like or (110)-like surfaces. The stronger coordination of TiCl4 on MgCl(OMe) would suggest a high Ti content of the catalyst. However,

Figure 5. Coordination of TiCl4 (top), dimethyl phthalate (middle), and ethyl benzoate (bottom) on (104)-like and (110)-like surfaces of MgCl(OMe). TiCl4 binds in the binuclear mode on (104)-like and in the mononuclear mode on (110)-like surfaces. Dimethyl phthalate binds in the bridging mode on (104)-like and in the chelate mode on (110)-like surfaces. Ethyl benzoate binds in the monodentate mode on (104)-like and in the dual monodentate mode on (110)-like surfaces.

Table 3. Stabilities of TiCl4/Donor Coordinated MgCl2 and MgCl(OMe) Surfaces Relative to the Respective Crystalline Monolayer and Free Adsorbatesa MgCl2

a

MgCl(OMe)

adsorbate

(104)

(110)

(104)-like

(110)-like

none TiCl4 dimethyl phthalate ethyl benzoate

13.5 −1.8 −11.1 −20.7

17.3 −2.5 −13.9 −20.3

15.7 −5.1 −2.6 −6.8

22.8 −7.7 −3.1 −7.1

The energies are given in kJ/mol per a surface length of Å.

MgCl(OMe)/TiCl4 catalyst contained only 2.1 wt % of titanium, which is a typical titanium content also in MgCl2supported catalysts.6,15 A relatively low titanium content of MgCl(OMe)/TiCl4 catalyst is probably due to residual 1,3dimethoxypropane present in the MgCl(OMe) support, which blocks part of the possible coordination sites. To conclude, the electron donor and TiCl4 binding properties of MgCl(OMe) differ notably from those of MgCl2. The stronger coordination of TiCl4 on MgCl(OMe) holds the potential for higher surface coverage of Ti and thus for a higher concentration of active centers after activation and a high activity in polymerization. On the other hand, the coordination of ester electron donors on MgCl(OMe) is weaker compared to MgCl2, suggesting that MgCl(OMe) would be a suitable support material for a catalyst in which the electron donors do not play a key role. E

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(2) Kashiwa, N. The Discovery and Progress of MgCl2-Supported TiCl4 Catalysts. J. Polym. Sci., Part A: Polym. Chem. 2004, 42, 1−8. (3) Stukalov, D. V.; Zakharov, V. A.; Potapov, A. G.; Bukatov, G. D. Supported Ziegler−Natta Catalysts for Propylene Polymerization. Study of Surface Species Formed at Interaction of Electron Donors and TiCl4 with Activated MgCl2. J. Catal. 2009, 266, 39−49. (4) Böhm, L. L. Ethylene Polymerization with Ziegler Catalysts: Fifty Years after the Discovery. Angew. Chem., Int. Ed. 2003, 42, 5010−5030. (5) Vanka, K.; Singh, G.; Iyer, D.; Gupta, V. K. DFT Study of Lewis Base Interactions with the MgCl2 Surface in the Ziegler−Natta Catalytic System: Expanding the Role of the Donors. J. Phys. Chem. C 2010, 114, 15771−15781. (6) Singh, G.; Kaur, S.; Makwana, U.; Patankar, R. B.; Gupta, V. K. Influence of Internal Donors on the Performance and Structure of MgCl2 Supported Titanium Catalysts for Propylene Polymerization. Macromol. Chem. Phys. 2009, 210, 69−76. (7) Vittadello, M.; Stallworth, P. E.; Alamgir, F. M.; Suarez, S.; Abbrent, S.; Drain, C. M.; Di Noto, V.; Greenbaum, S. G. Polymeric δMgCl2 Nanoribbons. Inorg. Chim. Acta 2006, 359, 2513−2518. (8) Di Noto, V.; Fregonese, D.; Marigo, A.; Bresadola, S. High Yield MgCl2-Supported Catalysts for Propene Polymerization: Effects of Ethyl Propionate as Internal Donor on the Activity and Stereospecificity. Macromol. Chem. Phys. 1998, 199, 633−640. (9) Fregonese, D.; Glisenti, A.; Mortara, S.; Rizzi, G. A.; Tondello, E.; Bresadola, S. MgCl2/TiCl4/AlEt3 Catalytic System for Olefin Polymerisation: A XPS Study. J. Mol. Catal. A: Chem. 2002, 178, 115−123. (10) Thushara, K. S.; Gnanakumar, E. S.; Mathew, R.; Jha, R. K.; Ajithkumar, T. G.; Rajamohanan, P. R.; Sarma, K.; Padmanabhan, S.; Bhaduri, S.; Gopinath, C. S. Toward an Understanding of the Molecular Level Properties of Ziegler−Natta Catalyst Support with and without the Internal Electron Donor. J. Phys. Chem. C 2011, 115, 1952−1960. (11) Brambilla, L.; Zerbi, G.; Piemontesi, F.; Nascetti, S.; Morini, G. Structure of Donor Molecule 9,9-Bis(Methoxymethyl)-Fluorene in Ziegler−Natta Catalyst by Infrared Spectroscopy and Quantum Chemical Calculation. J. Phys. Chem. C 2010, 114, 11475−11484. (12) Taniike, T.; Terano, M. Coadsorption Model for First-Principle Description of Roles of Donors in Heterogeneous Ziegler−Natta Propylene Polymerization. J. Catal. 2012, 293, 39−50. (13) Credendino, R.; Pater, J. T. M.; Correa, A.; Morini, G.; Cavallo, L. Thermodynamics of Formation of Uncovered and Dimethyl EtherCovered MgCl2 Crystallites. Consequences in the Structure of Ziegler−Natta Heterogeneous Catalysts. J. Phys. Chem. C 2011, 115, 13322−13328. (14) Potapov, A. G.; Bukatov, G. D.; Zakharov, V. A. DRIFT Study of Internal Donors in Supported Ziegler−Natta Catalysts. J. Mol. Catal. A: Chem. 2006, 246, 248−254. (15) Kissin, Y. V. Active Centers in Ziegler−Natta Catalysts: Formation Kinetics and Structure. J. Catal. 2012, 292, 188−200. (16) D’Amore, M.; Credendino, R.; Budzelaar, P. H. M.; Causá, M.; Busico, V. A Periodic Hybrid DFT Approach (Including Dispersion) to MgCl2-Supported Ziegler−Natta Catalysts − 1: TiCl4 Adsorption on MgCl2 Crystal Surfaces. J. Catal. 2012, 286, 103−110. (17) Andoni, A.; Chadwick, J. C.; Niemantsverdriet, H. J. W.; Thüne, P. C. A Flat Model Approach to Ziegler−Natta Catalysts for Propylene Polymerization and a Preparation Method of Well-defined Crystallites of MgCl2-supported Catalysts. Macromol. Symp. 2007, 260, 140−146. (18) Andoni, A.; Chadwick, J. C.; Niemantsverdriet, H. J. W.; Thüne, P. C. The Role of Electron Donors on Lateral Surfaces of MgCl2Supported Ziegler−Natta Catalysts: Observation by AFM and SEM. J. Catal. 2008, 257, 81−86. (19) Bazhenov, A.; Linnolahti, M.; Karttunen, A.; Pakkanen, T. A.; Denifl, P.; Leinonen, T. Modeling of Substitutional Defects in Magnesium Dichloride Polymerization Catalyst Support. J. Phys. Chem. C 2012, 116, 7957−7961. (20) Bazhenov, A.; Linnolahti, M.; Pakkanen, T. A.; Denifl, P.; Leinonen, T. Modeling the Stabilization of Surface Defects by Donors

Ethylene Polymerization. MgCl(OMe)/TiCl4 catalyst was preliminarily tested in ethylene polymerization. The activity of MgCl(OMe)/TiCl4 catalyst (2.1 wt % Ti) was 470 kgPE/(molTih) in our polymerization system. In comparison, the activity of MgCl2/TiCl4 reference catalyst (2.0 wt % Ti) prepared with the same method was only 190 kgPE/(molTih), giving almost 150% higher activity for the MgCl(OMe)-based catalyst.

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CONCLUSIONS The study revealed interesting aspects of the structure and catalytic properties of alkoxymagnesium chloride supports. Based on PXRD data and computational results, the structure of methoxymagnesium chloride closely resembles the layered structure of MgCl2. The computational results indicate that the formation of MgCl(OMe) is energetically feasible and suggest that methoxy groups of MgCl(OMe) favor positions close to each other, which would lead to an overall structure composed of alternating stripes of Mg(OMe)2 and MgCl2. The interaction of MgCl(OMe) with two electron donors (DIBP and EB) was investigated using both experimental and computational methods. The spectroscopic data indicated notably weaker interactions of DIBP and EB with MgCl(OMe) compared to MgCl2. Based on quantum chemical calculations, the unsaturated (104)-like and (110)-like surfaces of MgCl(OMe) are less stabilized on coordination of electron donors than the corresponding surfaces of MgCl2, supporting the experimental results. However, the coordination of TiCl4 stabilizes MgCl(OMe) significantly more than MgCl2. TiCl4 prefers coordination on the (110)-like surface of MgCl(OMe), whereas no particular preference in the case of electron donors was found. A MgCl(OMe)-supported catalyst was active in ethylene polymerization. The results obtained highlight the potential of MgCl(OMe) as a support material for Ziegler− Natta type polymerization catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b06488. PXRD data, IR spectra, and CP/MAS 13C NMR spectra of MgCl(OMe), MgCl(OMe)/TiCl4, and MgCl(OMe)/ electron donor complexes. Details of the computational methods and quantum chemical calculations concerning the structure of MgCl(OMe) and the interaction of MgCl(OMe) with electron donors and TiCl4 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: tuula.pakkanen@uef.fi. Tel.: +358 504354 379. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The computations were made possible by use of the Finnish Grid Infrastructure and Finnish Grid and Cloud Infrastructure resources.

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REFERENCES

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DOI: 10.1021/acs.jpcc.6b06488 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b06488 J. Phys. Chem. C XXXX, XXX, XXX−XXX